1.
Isomer
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An isomer is a molecule with the same molecular formula as another molecule, but with a different chemical structure. That is, isomers contain the number of atoms of each element. Isomers do not necessarily share similar properties, unless they also have the functional groups. There are two forms of isomerism, structural isomerism and stereoisomerism. In structural isomers, sometimes referred to as constitutional isomers, the atoms, Structural isomers have different IUPAC names and may or may not belong to the same functional group. For example, two position isomers would be 2-fluoropropane and 1-fluoropropane, illustrated on the side of the diagram above. In skeletal isomers the main chain is different between the two isomers. This type of isomerism is most identifiable in secondary and tertiary alcohol isomers, tautomers are structural isomers that spontaneously interconvert with each other, even when pure. They have different chemical properties and, as a consequence, distinct reactions characteristic to each form are observed, if the interconversion reaction is fast enough, tautomers cannot be isolated from each other. An example is when they differ by the position of a proton, such as in keto/enol tautomerism, there is, however, another isomer of C3H8O that has significantly different properties, methoxyethane. Unlike the isomers of propanol, methoxyethane has an oxygen connected to two carbons rather than to one carbon and one hydrogen. Methoxyethane is an ether, not an alcohol, because it lacks a hydroxyl group, propadiene and propyne are examples of isomers containing different bond types. Propadiene contains two double bonds, whereas propyne contains one triple bond, in stereoisomers the bond structure is the same, but the geometrical positioning of atoms and functional groups in space differs. This class includes enantiomers which are non-superposable mirror-images of each other, and diastereomers, enantiomers always contain chiral centers and diastereomers often do, but there are some diastereomers that neither are chiral nor contain chiral centers. Another type of isomer, conformational isomers, may be rotamers, diastereomers, for example, ortho- position-locked biphenyl systems have enantiomers. E/Z isomers, which have restricted rotation at a bond, are configurational isomers. They are classified as diastereomers, whether or not they contain any chiral centers, e/Z notation depicts absolute stereochemistry, which is an unambiguous descriptor based on CIP priorities. Cis–trans isomers are used to describe any molecules with restricted rotation in the molecule, for molecules with C=C double bonds, these descriptors describe relative stereochemistry only based on group bulkiness or principal carbon chain, and so can be ambiguous

2.
Retinal
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Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde, retinal is one of the many forms of vitamin A. Retinal is a chromophore, bound to proteins called opsins. Retinal allows certain microorganisms to convert light into metabolic energy, vertebrate animals ingest retinal directly from meat, or they produce retinal from carotenoids, either from one of two carotenes or from β-cryptoxanthin, a type of xanthophyll. These carotenoids must be obtained from plants or other photosynthetic organisms, no other carotenoids can be converted by animals to retinal, and some carnivores cannot convert any carotenoids at all. The other main forms of vitamin A, retinol, and an active form, retinoic acid. Invertebrates such as insects and squid use hydroxylated forms of retinal in their visual systems, living organisms produce retinal by irreversible oxidative cleavage of carotenoids. For example, beta-carotene + O2 →2 retinal catalyzed by a beta-carotene 15, 15-monooxygenase or a beta-carotene 15, vision begins with the photoisomerization of retinal. When the 11-cis-retinal chromophore absorbs a photon it isomerizes from the 11-cis state to the all-trans state, the absorbance spectrum of the chromophore depends on its interactions with the opsin protein to which it is bound, different opsins produce different absorbance spectra. Opsins are proteins and the visual pigments found in the photoreceptor cells in the retinas of eyes. An opsin is arranged into a bundle of seven transmembrane alpha-helices connected by six loops, in rod cells the opsin molecules are embedded in the membranes of the disks which are entirely inside of the cell. The N-terminus head of the molecule extends into the interior of the disk, in cone cells the disks are defined by the cells plasma membrane so that the N-terminus head extends outside of the cell. Retinal binds covalently to a lysine on the transmembrane helix nearest the C-terminus of the protein through a Schiff base linkage, formation of the Schiff base linkage involves removing the oxygen atom from retinal and two hydrogen atoms from the free amino group of lysine, giving H2O. Retinylidene is the divalent group formed by removing the oxygen atom from retinal, opsins are prototypical G protein-coupled receptors. Bovine rhodopsin, the opsin of the rod cells of cattle, was the first GPCR to have its X-ray structure determined, bovine rhodopsin contains 348 amino acid residues. The retinal chromophore binds at Lys296, although mammals use retinal exclusively as the opsin chromophore, other groups of animals additionally use four chromophores closely related to retinal. These are 3, 4-didehydroretinal, -3-hydroxyretinal, -3-hydroxyretinal, and -4-hydroxyretinal, many fish and amphibians use 3, 4-didehydroretinal, also called dehydroretinal. With the exception of the dipteran suborder Cyclorrhapha, the higher flies

3.
Rhodopsin
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Rhodopsin is a light-sensitive receptor protein involved in visual phototransduction. It is named after ancient Greek ῥόδον for “rose”, due to its pinkish color, Rhodopsin is a biological pigment found in the rods of the retina and is a G-protein-coupled receptor. Rhodopsin is extremely sensitive to light, and thus enables vision in low-light conditions, when rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 30 minutes, Rhodopsin was discovered by Franz Christian Boll in 1876. Rhodopsin consists of a protein moiety also called scotopsin, which binds covalently a cofactor called retinal, opsins are G protein coupled receptors and have seven transmembrane domains. The seven transmembrane domains form a pocket, where the retinal binds to a residue in the seventh transmembrane domain. The retinal lies horizontally to the cell membrane, and the cell membrane lipid bilayer embeds half of the rhodopsin. Thousands of rhodopsin molecules are found in outer segment disc of the host rod cell. Retinol is produced in the retina from Vitamin A, from dietary beta-carotene, Rhodopsin of the rods most strongly absorbs green-blue light and, therefore, appears reddish-purple, which is why it is also called visual purple. It is responsible for vision in the dark. Several closely related opsins exist that only in a few amino acids. Humans have eight different other opsins besides rhodopsin, as well as cryptochrome, the photopsins are found in the different types of the cone cells of the retina and are the basis of color vision. They have absorption maxima for yellowish-green, green, and bluish-violet light, the remaining opsin is found in photosensitive ganglion cells and absorbs blue light most strongly. In rhodopsin, the group of retinal is covalently linked to the amino group of a lysine residue on the protein in a protonated Schiff base. The intermediates formed during this process were first investigated in the laboratory of George Wald, the photoisomerization dynamics has been subsequently investigated with time-resolved IR spectroscopy and UV/Vis spectroscopy. A first photoproduct called photorhodopsin forms within 200 femtoseconds after irradiation and this intermediate can be trapped and studied at cryogenic temperatures, and was initially referred to as prelumirhodopsin. In subsequent intermediates lumirhodopsin and metarhodopsin I, the Schiffs base linkage to all-trans retinal remains protonated, the structure of rhodopsin has been studied in detail via x-ray crystallography on rhodopsin crystals. Several models attempt to explain how the group can change its conformation without clashing with the enveloping rhodopsin protein pocket

4.
Opsin
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Opsins are a group of light-sensitive proteins found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in image-forming. Opsins can be classified several ways, including function, type of chromophore, molecular structure, signal output, there are two groups of protein termed opsins. Type I opsins are employed by prokaryotes and by algae and fungi. No opsins have been found outside these groups, at one time it was thought that type I and type II were related because of structural and functional similarities. With the advent of genetic sequencing it became apparent that sequence identity was no greater than could be accounted for by random chance, however, in recent years new methods have been developed specific to deep phylogeny. Like type II opsins, type I opsins have a seven transmembrane domain structure similar to found in eukaryotic G-protein coupled receptors. Type I opsins are found in all three domains of life, Archaea, Bacteria, and Eukaryota, in Eukaryota, type I opsins are found mainly in unicellular organisms such as green algae, and in fungi. In most complex multicellular eukaryotes, type I opsins have been replaced with other molecules such as cryptochrome and phytochrome in plants. Microbial opsins are often known by the form of the molecule. Several type I opsins, such as proteo- and bacteriorhodopsin, are used by various groups to harvest energy from light to carry out metabolic processes using a non-chlorophyll-based pathway. Beside that, halorhodopsins of Halobacteria and channelrhodopsins of some algae, e. g. Volvox, serve them as light-gated ion channels, sensory rhodopsins exist in Halobacteria that induce a phototactic response by interacting with transducer membrane-embedded proteins that have no relation to G proteins. Type I opsins are used in optogenetics to switch on or off neuronal activity, type I opsins are preferred if the neuronal activity should be modulated at higher frequency, because they respond faster than type II opsins. Type II opsins are seven-transmembrane proteins belonging to the G protein-coupled receptor superfamily, type II opsins fall phylogenetically into four groups, C-opsins, Cnidops, R-opsins, and Go/RGR opsins. The Go/RGR opsins are divided into four sub-clades, Go-opsins, RGR, Peropsins, C-opsins, R-opsins, and the Go/RGR opsins are found only in Bilateria. Type II visual opsins are traditionally classified as either ciliary or rhabdomeric, ciliary opsins, found in vertebrates and cnidarians, attach to ciliary structures such as rods and cones. Rhabdomeric opsins are attached to light-gathering organelles called rhabdomeres and this classification cuts across phylogenetic categories so that both the terms ciliary and rhabdomeric can be ambiguous. Here, C-opsins refers to a clade found exclusively in Bilateria, similarly, R-opsin includes melanopsin even though it does not occur on rhabdomeres in vertebrates

5.
Visual perception
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Visual perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment. The resulting perception is known as visual perception, eyesight, sight. The visual system in animals allows individuals to assimilate information from their surroundings, the act of seeing starts when the cornea and then the lens of the eye focuses light from its surroundings onto a light-sensitive membrane in the back of the eye, called the retina. The retina is actually part of the brain that is isolated to serve as a transducer for the conversion of light into neuronal signals. These signals are processed via complex feedforward and feedback processes by different parts of the brain, signals from the retina can also travel directly from the retina to the superior colliculus. The perception of objects and the totality of the scene is accomplished by the visual association cortex. The visual association cortex combines all sensory information perceived by the cortex which contains thousands of modules that are part of modular neural networks. The neurons in the cortex send axons to the extrastriate cortex. The major problem in perception is that what people see is not simply a translation of retinal stimuli. Thus people interested in perception have long struggled to explain what visual processing does to create what is actually seen, there were two major ancient Greek schools, providing a primitive explanation of how vision is carried out in the body. The first was the theory which maintained that vision occurs when rays emanate from the eyes and are intercepted by visual objects. If an object was seen directly it was by means of coming out of the eyes. This theory was championed by scholars like Euclid and Ptolemy and their followers, the second school advocated the so-called intro-mission approach which sees vision as coming from something entering the eyes representative of the object. Plato makes this assertion in his dialogue Timaeus, as does Aristotle, alhazen carried out many investigations and experiments on visual perception, extended the work of Ptolemy on binocular vision, and commented on the anatomical works of Galen. Leonardo da Vinci is believed to be the first to recognize the special qualities of the eye. He wrote The function of the human eye, was described by a large number of authors in a certain way. But I found it to be completely different and his main experimental finding was that there is only a distinct and clear vision at the line of sight—the optical line that ends at the fovea. Although he did not use these words literally he actually is the father of the distinction between foveal and peripheral vision

6.
PubMed Identifier
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PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine at the National Institutes of Health maintains the database as part of the Entrez system of information retrieval, from 1971 to 1997, MEDLINE online access to the MEDLARS Online computerized database primarily had been through institutional facilities, such as university libraries. PubMed, first released in January 1996, ushered in the era of private, free, home-, the PubMed system was offered free to the public in June 1997, when MEDLINE searches via the Web were demonstrated, in a ceremony, by Vice President Al Gore. Information about the journals indexed in MEDLINE, and available through PubMed, is found in the NLM Catalog. As of 5 January 2017, PubMed has more than 26.8 million records going back to 1966, selectively to the year 1865, and very selectively to 1809, about 500,000 new records are added each year. As of the date,13.1 million of PubMeds records are listed with their abstracts. In 2016, NLM changed the system so that publishers will be able to directly correct typos. Simple searches on PubMed can be carried out by entering key aspects of a subject into PubMeds search window, when a journal article is indexed, numerous article parameters are extracted and stored as structured information. Such parameters are, Article Type, Secondary identifiers, Language, publication type parameter enables many special features. As these clinical girish can generate small sets of robust studies with considerable precision, since July 2005, the MEDLINE article indexing process extracts important identifiers from the article abstract and puts those in a field called Secondary Identifier. The secondary identifier field is to store numbers to various databases of molecular sequence data, gene expression or chemical compounds. For clinical trials, PubMed extracts trial IDs for the two largest trial registries, ClinicalTrials. gov and the International Standard Randomized Controlled Trial Number Register, a reference which is judged particularly relevant can be marked and related articles can be identified. If relevant, several studies can be selected and related articles to all of them can be generated using the Find related data option, the related articles are then listed in order of relatedness. To create these lists of related articles, PubMed compares words from the title and abstract of each citation, as well as the MeSH headings assigned, using a powerful word-weighted algorithm. The related articles function has been judged to be so precise that some researchers suggest it can be used instead of a full search, a strong feature of PubMed is its ability to automatically link to MeSH terms and subheadings. Examples would be, bad breath links to halitosis, heart attack to myocardial infarction, where appropriate, these MeSH terms are automatically expanded, that is, include more specific terms. Terms like nursing are automatically linked to Nursing or Nursing and this important feature makes PubMed searches automatically more sensitive and avoids false-negative hits by compensating for the diversity of medical terminology. The My NCBI area can be accessed from any computer with web-access, an earlier version of My NCBI was called PubMed Cubby

7.
Eye
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Eyes are organs of the visual system. They provide organisms vision, the ability to process visual detail, eyes detect light and convert it into electro-chemical impulses in neurons. Eyes with resolving power have come in ten different forms. Image-resolving eyes are present in molluscs, chordates and arthropods, the simplest eyes, such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. Complex eyes can distinguish shapes and colours, the visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception. In other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, the first proto-eyes evolved among animals 600 million years ago about the time of the Cambrian explosion. The last common ancestor of animals possessed the biochemical toolkit necessary for vision, in most vertebrates and some molluscs, the eye works by allowing light to enter and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells and the rod cells in the retina detect, the visual signals are then transmitted to the brain via the optic nerve. The eyes of most cephalopods, fish, amphibians and snakes have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses. Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, each sensor has its own lens and photosensitive cell. Some eyes have up to 28,000 such sensors, which are arranged hexagonally, compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have eyes of only a few facets, each with a retina capable of creating an image. With each eye viewing a different thing, an image from all the eyes is produced in the brain, providing very different. Possessing detailed hyperspectral colour vision, the Mantis shrimp has been reported to have the worlds most complex colour vision system, trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes, in this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however, some trilobites had only one, in contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a pair of simple eyes with a narrow field of view. Some insect larvae, like caterpillars, have a different type of simple eye which gives a rough image, some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually see in the normal sense

Isomer
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An isomer is a molecule with the same molecular formula as another molecule, but with a different chemical structure. That is, isomers contain the number of atoms of each element. Isomers do not necessarily share similar properties, unless they also have the functional groups. There are two forms of isomerism, structural isomerism and stereoisomeri

Retinal
–
Retinal is also known as retinaldehyde. It was originally called retinene, and renamed after it was discovered to be vitamin A aldehyde, retinal is one of the many forms of vitamin A. Retinal is a chromophore, bound to proteins called opsins. Retinal allows certain microorganisms to convert light into metabolic energy, vertebrate animals ingest ret

Rhodopsin
–
Rhodopsin is a light-sensitive receptor protein involved in visual phototransduction. It is named after ancient Greek ῥόδον for “rose”, due to its pinkish color, Rhodopsin is a biological pigment found in the rods of the retina and is a G-protein-coupled receptor. Rhodopsin is extremely sensitive to light, and thus enables vision in low-light condi

1.
Three-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red.

Opsin
–
Opsins are a group of light-sensitive proteins found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in ima

1.
Three-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red.

Visual perception
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Visual perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment. The resulting perception is known as visual perception, eyesight, sight. The visual system in animals allows individuals to assimilate information from their surroundings, the act of seeing start

1.
Leonardo da Vinci: The eye has a central line and everything that reaches the eye through this central line can be seen distinctly.

2.
The visual dorsal stream (green) and ventral stream (purple) are shown. Much of the human cerebral cortex is involved in vision.

3.
Eye movement first 2 seconds (Yarbus, 1967)

4.
Vision

PubMed Identifier
–
PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine at the National Institutes of Health maintains the database as part of the Entrez system of information retrieval, from 1971 to 1997, MEDLINE online access to the

1.
PubMed

Eye
–
Eyes are organs of the visual system. They provide organisms vision, the ability to process visual detail, eyes detect light and convert it into electro-chemical impulses in neurons. Eyes with resolving power have come in ten different forms. Image-resolving eyes are present in molluscs, chordates and arthropods, the simplest eyes, such as those in